Respect unavailable AI chat client providers (#8453)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: Evangelink <11340282+Evangelink@users.noreply.github.com>

Author: Copilot

src/Platform/Microsoft.Testing.Platform.AI/ChatClientProviderExtensions.cs

@@ -33,11 +33,11 @@ public static void AddChatClientProvider(this ITestApplicationBuilder testApplic
     /// </summary>
     /// <param name="serviceProvider">The service provider.</param>
     /// <param name="cancellationToken">The cancellation token.</param>
-    /// <returns>A task representing the asynchronous operation that returns an instance of <see cref="IChatClient"/>.</returns>
+    /// <returns>A task representing the asynchronous operation that returns an instance of <see cref="IChatClient"/>, or <see langword="null"/> when no available provider is registered.</returns>
     public static async Task<IChatClient?> GetChatClientAsync(this IServiceProvider serviceProvider, CancellationToken cancellationToken)
     {
         var provider = (IChatClientProvider?)serviceProvider.GetService(typeof(IChatClientProvider));
-        return provider is null
+        return provider is null || !provider.IsAvailable
             ? null
             : await provider.CreateChatClientAsync(cancellationToken).ConfigureAwait(false);
     }

test/UnitTests/Microsoft.Testing.Platform.UnitTests/AI/ChatClientProviderExtensionsTests.cs

@@ -0,0 +1,56 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+using Microsoft.Extensions.AI;
+using Microsoft.Testing.Platform.AI;
+using Microsoft.Testing.Platform.Services;
+
+namespace Microsoft.Testing.Platform.UnitTests.AI;
+
+#pragma warning disable TPEXP // Type is for evaluation purposes only and is subject to change or removal in future updates. Suppress this diagnostic to proceed.
+
+[TestClass]
+public sealed class ChatClientProviderExtensionsTests
+{
+    [TestMethod]
+    public async Task GetChatClientAsync_WhenProviderIsUnavailable_ReturnsNull()
+    {
+        ServiceProvider serviceProvider = new();
+        UnavailableChatClientProvider provider = new();
+        serviceProvider.AddService(provider);
+
+        IChatClient? chatClient = await serviceProvider.GetChatClientAsync(CancellationToken.None).ConfigureAwait(false);
+
+        Assert.IsNull(chatClient);
+        Assert.AreEqual(1, provider.IsAvailableCallCount);
+        Assert.AreEqual(0, provider.CreateChatClientAsyncCallCount);
+    }
+
+    private sealed class UnavailableChatClientProvider : IChatClientProvider
+    {
+        public int IsAvailableCallCount { get; private set; }
+
+        public int CreateChatClientAsyncCallCount { get; private set; }
+
+        public bool IsAvailable
+        {
+            get
+            {
+                IsAvailableCallCount++;
+                return false;
+            }
+        }
+
+        public bool HasToolsCapability => false;
+
+        public string ModelName => "Unavailable";
+
+        public Task<IChatClient> CreateChatClientAsync(CancellationToken cancellationToken)
+        {
+            CreateChatClientAsyncCallCount++;
+            throw new InvalidOperationException("CreateChatClientAsync should not be called for unavailable providers.");
+        }
+    }
+}
+
+#pragma warning restore TPEXP

test/UnitTests/Microsoft.Testing.Platform.UnitTests/Microsoft.Testing.Platform.UnitTests.csproj

@@ -39,6 +39,12 @@
     <PackageReference Include="Moq" />
   </ItemGroup>
 
+  <ItemGroup>
+    <ProjectReference Include="$(RepoRoot)src\Platform\Microsoft.Testing.Platform.AI\Microsoft.Testing.Platform.AI.csproj">
+      <SetTargetFramework Condition="$([MSBuild]::GetTargetFrameworkIdentifier('$(TargetFramework)')) != '.NETCoreApp'">TargetFramework=netstandard2.0</SetTargetFramework>
+    </ProjectReference>
+  </ItemGroup>
+
   <ItemGroup>
     <Compile Include="$(RepoRoot)src/Polyfills/**/*.cs" Link="Polyfills\%(RecursiveDir)%(Filename)%(Extension)" />
   </ItemGroup>